This invention relates generally to measurement of relative velocity between a wave source-sensor combination and a field of scatterers, and more particularly to a velocity measuring sonar system for measuring the speed of a surface or submerged marine vessel relative either to the bottom or to the water mass at a predetermined range or depth below the vessel.
Conventionally, marine velocity measuring systems based on sonar concepts have used doppler techniques commonly employing four sonar emitters and receivers two of which are angled outwardly with respect to the vertical in the fore-and-aft plane and two similarly angled in the athwartship plane. Motion of the vessel then results in a doppler shift in frequency of the echo or signal return in each of the beams. For example, in the case of forward motion of the vessel there will be an upward doppler shift of the fore-directed beam echo and a downward shift of freqency of the aft-directed beam echo. Similar doppler shifts occur in the case of "leeway" or transverse motion of the vessel, thus enabling velocity measurement in both fore-and-aft and athwartship directions.
Doppler sonar velocity measuring systems of this general kind have come into wide use and function very effectively for many marine logging applications. They are subject to certain limitations, however, which tend to compromise their utility in others. Among these limitations are the sensitivity of the beam direction and hence the calibration to the bottom scattering characteristics which may be different in one area than in another, and the sensitivity of the calibration also to variations in the local sound velocity due to variations in water temperature, for example. Further, because of the narrow beams required, there may be difficulties of insuring overlap of transmit and receive patterns when large pitch and roll rates and long propagation delays are encountered. Other limitations derive from the rather stringent requirements as to transmit-receive beam pattern and the relatively large dimensions of the transmit-receive arrays necessary to achieve these patterns.
An entirely different approach to velocity measurement was disclosed by the present inventor some years ago in U.S. Pat. No. 3,147,477--Dickey. That patent first disclosed the use of correlation measurements to estimate the velocity of a source/sensor platform relative to an ensemble of fixed scatters. The patent is most detailed in its description of an aircraft ground speed measurement system, using radar or radio frequency waves for velocity determination, but the patent also discloses the measurement of speed of surface ships and submarines using sound waves. Operation of the correlation sensor is basically the same, as explained in the patent, irrespective of the frequency of the wave energy used or the composition of the medium through which it is transmitted and received.
Correlation sonar operates on the unique character of acoustic energy that has been back-scattered from an ensemble of scatters formed either by the bottom or by a volume of water intermediate the ship and the bottom. These scatterers are illuminated by the beam from a sonar transmitter directed vertically downward toward the bottom, and reflected back to two or more similarly directed receivers carried by the ship adjacent to the transmitter.
The echo or reflected energy returned from such myriad of bottom scatterers forms an interference field in the vicinity of the ship. If the ship is stationary and transmitting a continuous waveform, the field will be stationary and continuous. If the ship is moving, the continuous field will appear from the ship to be moving backward relative to the ship at twice the ship's actual velocity. This field motion can most easily be explained by noting that, as the ship moves, the range to the scatterers forward of the ship is decreasing while the range to the aft-scatterers is increasing, producing the effect of rotating the surface containing the scatterers about a point directly beneath the ship. This produces the apparent backward field motion.
A pair of downward-directed receiving hydrophones spaced along the longitudinal axis of the ship will sense the field, producing identical output waveforms except that the aft hydrophone's output will be delayed by an amount (T.sub.d) equal to the spacing between hydrophones divided by twice the velocity (V) of the vessel, in accordance with the relation: T.sub.d =s/2V. Thus by holding either the time delay or the hydrophone separation distance fixed while adjusting the other of these parameters in a manner to achieve maximum correlation, as indicated by identical or substantially identical output waveforms at the two hydrophones, and measuring the value of the variable at which this is achieved, the vessel velocity may readily be determined from the foregoing relation between these parameters.
If instead of a continuous transmitted waveform, pulsed transmissions are used, a similar field will be formed but it will display a time-varying characteristic as well as a spatially varying one. This enables correlation of signals received at the spaced hydrophones in any of several different ways, as will be further detailed hereinafter.
The correlation velocity sensor of the present invention is based on the principle of the aforesaid Dickey patent, and is directed more specifically to correlation velocity sensors for marine applications employing sonic or ultrasonic wave energy and a plurality of receiver transducers disposed in a fixed two-dimensional array. Measurement of the several components of relative velocity then is accomplished by cross-correlation measurements between the various echoes received at the several receiver transducers over a time interval.
The correlation velocity sensor of the present invention affords significant advantages both over doppler systems and over other known correlation sonar systems. This sensor can be designed to operate in any water depth, since requirements on beamwidth, propagation loss, and roll-pitch rates are not limiting. The calibration and accuracy of velocity measurement, insofar as concerns the horizontal velocity components, depends only on measured values of the spacings between transducers in the receive array and measured values of time, and is independent of sound propagation speed and of bottom scattering characteristics. Also, the use of a wide transmit bandwidth provides a larger time-bandwidth product for smoothing statistical fluctuations than usually is realizable in the doppler case.
These and other features and advantages of the correlation velocity sensor of the present invention will become more apparent in the light of the detailed description of the invention hereinafter.